Waveguide photodetector with integrated electronics

ABSTRACT

A germanium on silicon waveguide photodetector disposed on a silicon on insulator (SOI) substrate. The photodetector is incorporated into a section of a planar silicon waveguide on the substrate. The photodetector generates an electric current as an infrared optical signal travels through the photodetector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/600,563 filed on Jun. 19, 2003 now U.S. Pat. No. 7,453,132, whichclaims priority from U.S. Provisional applications No. 60/389,962 filedJun. 19, 2002, No. 60/391,277 filed Jun. 24, 2002, No. 60/432,925 filedDec. 12, 2002 and No. 60/433,470 filed Dec. 13, 2002.

FIELD OF THE INVENTION

The present invention relates to a semiconductor waveguide photodetectorand its use in a monolithic integrated circuit for the conversion ofoptical signals to electrical signals.

BACKGROUND OF THE INVENTION

The rapid expansion in the use of the Internet has resulted in a demandfor high speed communications links and devices, including optical linksand devices. Optical links using fiber optics have many advantagescompared to electrical links: large bandwidth, high noise immunity,reduced power dissipation and minimal crosstalk. Optoelectronicintegrated circuits made of silicon are highly desirable since theycould be fabricated in the same foundries used to make VLSI integratedcircuits. Optical communications technology is typically operating inthe 1.3 μm and 1.55 μm infrared wavelength bands. The optical propertiesof silicon are well suited for the transmission of optical signals, dueto its transparency in the infrared wavelength bands of 1.3 μm and 1.55μm and its high refractive index. As a result, low loss planar siliconoptical waveguides have been successfully built.

A silicon based waveguide is just one of many components needed to makean integrated optoelectronic circuit. An optical signal received by anoptoelectronic circuit has in many cases to be converted to anelectronic signal for further processing by electronic circuits.Conversion of optical signals to electronic signals can be achieved by aphotodetector. Silicon, due to its bandgap of 1.12 eV, cannot be used tomake photodetectors for infrared band signals, because it is transparentto light at wavelengths above 1.1 μm. Silicon's transparency to infraredlight makes it ideal for use as a planar waveguide on an integratedcircuit, but eliminates it from use as an infrared photodetector.

Hybrid and integrated optoelectronics have been built withphotodetectors made of III-V type semiconductors such as InGaAs, butthese devices are difficult to integrate into a silicon chip. SiGealloys have potential as infrared photodetectors, but primarily in the1.3 μm wavelength band. Germanium in bulk form has been used as aphotodetector in the infrared band, due to its bandgap of 0.66 eV.Making a germanium on silicon photodetector has been difficult, due tothe large lattice mismatch, of about 4% for pure germanium on silicon.

After conversion of an optical signal to an electronic signal, the lowlevel electrical signal needs to be amplified and processed byassociated electronic circuits. The electronic circuits are typicallybuilt on a silicon chip. Due to the lack of useable silicon based IRphotodetectors, such optoelectronic conversion is typically performed byhybrid circuits, which are much more expensive than monolithicintegrated circuits.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a germanium on siliconwaveguide photodetector disposed on a silicon on insulator (SOI)substrate. The photodetector is incorporated into a section of a planarsilicon waveguide on the substrate. The photodetector generates anelectric current as an infrared optical signal travels through thephotodetector.

An alternate embodiment of the present invention is a chip forintegrating an optoelectronic converter. An optical signal is connectedto a planar waveguide on the chip through an optical coupling. Thewaveguide has along its length, a germanium on silicon waveguidephotodetector, which generates an electric current as an infraredoptical signal propagates through the photodetector. Electroniccircuits, such as CMOS circuits, on the same chip convert the electriccurrent to a digital signal. The digital signal can be further processedin the digital domain on the same chip or sent to other chips.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a waveguide photodetector, according to oneembodiment of the present invention.

FIG. 2 is a top view of a waveguide photodetector, according to oneembodiment of the present invention.

FIG. 3 is a top view of a waveguide photodetector, according to analternate embodiment of the present invention.

FIG. 4 is a block diagram of the operation of a waveguide photodetector,according to one embodiment of the present invention.

FIG. 5 is a side view of a waveguide photodetector, according to anotherembodiment of the present invention.

FIG. 6 is a top view of a waveguide photodetector, according to anadditional embodiment of the present invention.

FIG. 7 is a top view of a waveguide photodetector, according to analternate embodiment of the present invention.

FIG. 8 is a side view of a waveguide photodetector integrated with aCMOS transistor, according to one embodiment of the present invention.

FIG. 9 is a block diagram of the operation of an integrated, monolithicoptoelectronic converter, according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is a side view of a waveguide photodetector 100, not to scale,according to a preferred embodiment of the present invention. Thesubstrate 101 is made of silicon dioxide layer 103 on top of siliconlayer 102. A silicon planar waveguide 104 receives an optical signal 115from a source, not shown, to the left of the photodetector.

The waveguide 104 is made of a silicon core 105 and surrounded bycladding in the form of one or more dielectric layers. The claddingincludes the silicon dioxide layer 103 underneath the silicon core andthe dielectric films 106. On top of the waveguide core 105, is a layerof germanium 107. A germanium on silicon heterojunction is formed by thegermanium layer 107 on top of the silicon core 105 of the waveguide 104.The waveguide core and heterojunction are surrounded on their sides andfrom above by one or more dielectric cladding layers 106. The dielectriclayers 106 can also be referred to as inter-layer dielectric films. Someof the dielectric layers are typically made of silicon dioxide.

The germanium layer 107 is preferably P doped. The silicon core 105 ofthe waveguide 104 includes a preferably N doped region 112. A depletionregion 114 is made of germanium depletion region 110 and silicondepletion region 111.

Ohmic contacts 116 and 117, typically of metal silicide, such as cobaltsilicide are fabricated, respectively in the germanium 107 and silicon112. Conductive plug 108 connects the ohmic contact 116 to the metalsegment 118, which is part of the first metal layer (M1). Conductiveplug 109 connects the ohmic contact 117 to the metal segment 119, whichis part of the first metal layer (M1). Each of the conductive plugs 109and 108 can be each made of multiple conductive plugs. Each of themultiple conductive plugs can be connected to the first metal layer ofan integrated circuit.

A particularly advantageous aspect of the waveguide photodetectors ofthe present invention is the increased electric current generated due tothe use of multiple conductive plugs 109 to the ohmic contact 116 on thegermanium 107 and the use of multiple conductive plugs 110 to the ohmiccontact 117 on the silicon 112.

In one embodiment of the present invention, during fabrication of thephotodetector 100, an N type impurity is introduced into region 112 ofthe silicon core 105 of the waveguide 104; and P type dopants areintroduced into region 110 of the germanium layer 107. The N dopedregion 112 extends to the left and to the right of the top surface ofthe waveguide core 105 in contact with the germanium layer 107. The Ndoped region 112 of the waveguide preferably extends far enough to theright of the germanium 107, opposite the light input, to provide spacefor the ohmic contacts 117.

A germanium on silicon heterojunction is formed by the contact of thegermanium P layer 107 with the silicon N region 112. The germanium onsilicon heterojunction forms a waveguide core which receives an opticalsignal at its input from the waveguide 104. Depletion region 114 can bemade of two layers: the germanium depletion region 110 and the silicondepletion region 111. The depletion region 114 has no charge carriersand an electric field exists within the depletion region 114. Asinfrared light 115 travels through the waveguide photodetector 100 fromleft to right, some of the light enters into the germanium layer 107.The silicon depletion region 111 is transparent to infrared but thegermanium depletion region 110 absorbs infrared light.

When infrared light enters into the germanium depletion region 110, theabsorption of the photons generates electron hole pairs. Within thedepletion region 114, there is an electric field which separates thephoton generated electron hole pairs. The holes travel into the P dopedregion from the depletion region 114. The electrons travel into the Ndoped region from the depletion region 114. The motion of the holes andelectrons out of the depletion region 114 causes a current to flow inthe photodetector 100 and through ohmic contacts 116 and 117. Theresulting electric current is proportional to the intensity of theinfrared light 115 traveling through the waveguide 104. Thephotodetector 100 operates as a pin diode and is typically reversebiased.

The collection efficiency of the photodetector can be improved bydelivering as much of the power of the optical signal into the germaniumdepletion region 110, where photons are converted into electron holepairs.

Waveguide 104 in a preferred embodiment is typically 0.5 μm wide and 0.2μm high. The waveguide width can vary from 0.1 to 10.0 μm. The waveguideheight can vary from 0.1 μm to 1.0 μm.

The large difference between the high refractive index (n) of about 3.5for silicon as compared to the much lower refractive index of about 1.5for silicon dioxide, confines the light to the core with minimal leakagethrough the cladding, provides a very small bending radius of about 10μm and thus saves space on the chip.

The germanium 107 in a preferred embodiment is typically made of a 200nm layer of monocrystalline germanium, which is 0.5 μm wide and 25 μmlong. The monocrystalline germanium layer can have a thickness of from10 nm to 1000 nm. The germanium 107 can be narrower or wider than thewaveguide core 105 underneath the germanium. In alternate embodiments,the germanium 107 is made of polycrystalline germanium.

The depletion region 114 in a preferred embodiment typically has athickness of 30 nm. The depletion region 114 can vary from 0.0 μm to 1.0μm in thickness. The small size of the total junction area and thenarrow thickness of the depletion region 114 reduces the capacitance ofthe junction, and makes practical high speed operation.

The waveguide 104 and the germanium 107 together form waveguidephotodetector 100 with a combined optical thickness of the core 105 andthe germanium 107. The optical thickness of the waveguide is increased,because of the transmission of light into the germanium 107 from thesilicon waveguide 104. Germanium has a refractive index of about 4.2 andthe silicon core 105 has a refractive index of about 3.5. As lightpropagates through the core 105 next to the germanium 107, some lightpasses into the germanium 107, because the refractive indices of thegermanium 107 and silicon core 105 are close enough to allow light topass through a germanium—silicon boundary.

When a single mode optical signal reaches the input or leading edge ofthe photodetector 100, the optical signal is transformed into multiplemodes, due to the transmission of light into the germanium 107.

FIG. 2 is a top view of a waveguide photodetector 200, not to scale,according to an embodiment of the present invention. Optical signal 215is propagated in waveguide 204. A plurality of conductive plugs 208connect to an ohmic contact in germanium 207. A plurality of conductiveplugs 209 are made into an ohmic contact in the doped region of thesilicon core 205 of the waveguide 204 on the far side of thephotodetector 200. To increase the sensitivity of the photodetector 200,a plurality of conductive plugs 208 and 209 are used to increase theamount of electric current picked up from the photodetector 200. Thegermanium 207 can be narrower or wider than the width of the waveguide204.

FIG. 3 is a top view of a waveguide photodetector 300, not to scale,according to a preferred embodiment of the present invention. Opticalsignal 315 is propagated in waveguide 304, which widen as it approachesgermanium 307. To simplify the fabrication of the photodetector 300, itand the waveguide 304 are made much wider than the typical width of 0.5μm of the straight section of waveguide 304. Waveguide 304 at its widestpoint in the photodetector region can be several times the width of thestraight section of the waveguide 304. A plurality of conductive plugs308 are made into germanium 307. A plurality of conductive plugs 309 aremade into a doped region of the silicon core 305 of the waveguide 304 onthe far side of the photodetector 300, furthest away from the input forthe optical signal.

Light is attenuated as it travels through the heterojunction of thephotodetector 300. Placing an ohmic contact at the far end of thephotodetector 300 prevents its interfering with the propagation of lightinto the photodetector. The conductive plugs 308 and 309 to the ohmicregion are positioned away from the input into the photodetector toprevent the attenuation of the input signal. The conductive plugs 308and 309 are also positioned to avoid the area of light penetration intothe photodetector. If any of the conductive plugs 308 or 309 are withinthe area of light penetration in the photodetector, they will attenuatethe further transmission of light, which will reduce the electriccurrent generated by the photodetector 300.

FIG. 4 is a block diagram of the operation of a waveguide photodetectoraccording to an embodiment of the present invention. An optical inputsignal 401 is connected to input waveguide 402, which provides opticalsignal 403 to mode converter 404. Mode converter 404 processes opticalsignal 403 into modified optical signal 405, which is input to thegermanium on silicon heterojunction 406. Germanium on siliconheterojunction 406 absorbs the modified optical signal 405 and generateselectrical signal 407. By processing the optical signal 403, the modeconverter 404 can optimize the collection efficiency of the germanium onsilicon heterojunction 406.

The designs of waveguides and waveguide photodetectors can be optimizedto minimize reflections, dispersions and losses. The mode converter 404can be designed to minimize the reflections that would otherwise takeplace at the input to the heterojunction 406. The mode converter 404 canalso be designed to make the best possible match in terms of opticalmodes between the optical signal 403 from the input waveguide 402 andthe heterojunction 406.

The input waveguide 402 can be of the single mode or multiple mode type.The impact of a waveguide upon the mode of an optical signal can beanalyzed by determining the effect of the waveguide on the: number ofmodes, the modal pattern and the polarization pattern, as compared tothe optical signal at the input. A mode converter 404 of the presentinvention can optimize the modal characteristics of the signal 403 sentto the heterojunction 406, so as to send the maximum level of power intothe germanium depletion region 110, as discussed with regard to FIG. 1.A single mode waveguide 401 is the preferred type for use with aphotodetector of the present invention. If the optical signal 403 sentinto the heterojunction 406 has a single optical mode, then the modeconverter 404 can control the formation of multiple optical modes insuch a way as to align the strongest mode generated with the lightabsorbing germanium depletion region of the heterojunction 406.

FIG. 5 is a side view of a waveguide photodetector 500, not to scale,according to an alternate embodiment of the present invention. Thesubstrate 501 is made of silicon dioxide layer 503 on top of siliconlayer 502. A silicon planar waveguide 504 receives an optical signal 515from a source, not shown, to the left of the photodetector.

The waveguide 504 is made of a silicon core 505 surrounded by claddingin the form of one or more dielectric layers, including the silicondioxide layer 503 underneath the silicon core. On top of silicon core505 of waveguide 504 is a layer of germanium 507. The silicon core ofthe waveguide 504 and the germanium 507 are surrounded on their sidesand from above by dielectric layer 506.

Conductive plug 508 extends from metal segment 518 of the first metallayer on top of the dielectric layers 506 to an ohmic contact 516 in thegermanium layer 507. Conductive plug 509 extends from metal segment 519of the first metal layer on top of the dielectric layers 506 to an ohmiccontact 517 in the doped region 512 of the waveguide core 505. Eachconductive plug 508 and 509 can be a plurality of conductive plugs, inorder to provide the maximum pickup of electric current.

In front of the germanium 507, a mode converter 520 is disposed on topof waveguide core 505. The mode converter 520 can be made of a layer ofdielectric, such as polysilicon (polycrystalline silicon). The largedifference in refractive index between the silicon waveguide core 505 (nequal to about 3.5) and the silicon dioxide 503 (n equal to about 1.5)underneath the waveguide core 505 and the surrounding dielectriccladdings, causes an optical signal in the waveguide core 505 to benarrowly confined. When the optical signal reaches the input or theleading edge of the germanium 507, there can be many reflectionsgenerated at the leading edge of the germanium 507, due to the abruptchange in refractive index. Depositing the mode converter 520 made ofpolysilicon (n equal to about 3.6) in front of the germanium 507 (nequal to about 4.2) provides a much smoother optical transition andreduces the generation of reflections at the input to the germanium 507layer.

FIG. 6 is a top view of a waveguide photodetector 600, not to scale,according to an alternate embodiment of the present invention. Opticalsignal 615 is propagated in waveguide 604, which widen as it approachesgermanium 607. To simplify the fabrication of the photodetector 600, itand the waveguide 604 are made much wider than the typical width of 0.5μm of the straight waveguide 604. Waveguide 604 at its widest point inthe photodetector region can be several times the typical width of thestraight section of the waveguide 604. A plurality of conductive plugs608 are made into an ohmic contact in the germanium 607. A plurality ofconductive plugs 609 are made into an ohmic contact in the doped regionof the silicon layer on the far side of the photodetector 600, furthestaway from the input for the optical signal.

In front of the germanium 607, a mode converter 620 is disposed on topof waveguide core 605. The mode converter 620 can be made of a layer ofdielectric, such as polysilicon (polycrystalline silicon). Depositingthe mode converter 620 made of polysilicon (n equal to about 3.6) infront of the germanium 607 (n equal to about 4.2) provides a muchsmoother optical transition and reduces the generation of reflections atthe input to the germanium 607 layer.

FIG. 7 is a top view of a waveguide photodetector 700, not to scale,according to another embodiment of the present invention. Optical signal715 is propagated in waveguide 704, which starts to widen as itapproaches germanium 707. To simplify the fabrication of thephotodetector 700, it and the waveguide 704 are made much wider than thetypical width of 0.5 μm of the straight waveguide 704. Waveguide 704 atits widest point in the photodetector region can be several times thewidth of the straight section of the waveguide 704. A plurality ofconductive plugs 708 are made into an ohmic region in germanium 707. Aplurality of conductive plugs 709 are made into an ohmic contact in thedoped region of the silicon core 705 on the far side of thephotodetector 700, furthest away from the input for the optical signal.

Close to the leading edge of the germanium 707, where an optical signalfirst enters the germanium 707 from the waveguide 704, a plurality ofstructures 725 are formed and filled with a dielectric such as silicondioxide. The structures 725 can be of a variety of shapes, such ascylinders, cones or other shapes. Placing silicon dioxide structures 725with a lower refractive index (n equal to about 1.5) into the germanium(n equal to about 3.5) changes the refractive index in that region to avalue somewhere between the two indices and provides for a smootheroptical transition for the optical signal propagating into the germanium707. The gradual change in refractive index will reduce the number andamplitude of the reflections generated at the boundary with thegermanium 707.

The plurality of structures 725 can be placed in a number of rows andthe structures 725 can typically be 300 nm in diameter and be spaced ata distance of 500 nm between their approximate center lines. Eachsuccessive row of structures 725 can be thinner and spaced farther apartto provide a gradual transition. The size and arrangement of thestructures 725 need not be in a regular pattern, but can be irregular orrandom, as may be needed to obtain a smooth optical transition.

FIG. 8 is a side view of a waveguide photodetector integrated with aCMOS transistor, according to one embodiment of the present invention.Optoelectronic circuit 800 is comprised of waveguide photodetector 835and CMOS transistor 830. The substrate 801 is made of silicon dioxidelayer 803 on top of silicon layer 802. A silicon planar waveguide 804receives an optical signal 815 from a source, not shown, to the left ofthe photodetector.

The waveguide 804 is made of silicon core 805 surrounded by cladding inthe form of one or more dielectric layers, including the silicon dioxidelayer 803 underneath the silicon core. On top of silicon core 805 is alayer of germanium 807. The germanium on silicon heterojunction issurrounded on its sides and from above by cladding consisting of one ormore dielectric layers 806. The dielectric layers 806 can also bereferred to as inter-layer dielectric (ILD) films.

An ohmic contact 816 is implanted into the germanium 807. A conductiveplug 808 extends from metal segment 818 of the first metal layer on topof the dielectric layers 806 to the ohmic contact 816. An ohmic contact817 is made of metal silicide in the doped region 812 of the siliconcore 805. Conductive plug 809 extends from metal segment 819 of thefirst metal layer on top of the dielectric layers 806 to ohmic contact817. Conductive plugs 808 and 809 can be made of tungsten or otherconductive material. Each conductive plug 808 and 809 is a plurality ofconductive plugs, in order to provide for the maximum pickup of electriccurrent.

In front of the germanium 807, a layer of dielectric such as polysilicon820 is disposed on top of waveguide 804. The polysilicon 820 layer infront of the germanium 807 operates as a mode converter and provides fora smoother optical transition and reduces the generation of reflectionsat the start of the germanium 807 layer.

CMOS transistor 830 is comprised of silicon body 840, which includes adoped region 853 for a drain, polysilicon region 854 for a gate anddoped region 855 for a source. Ohmic contacts 846 are disposed into theregions 853, 854 and 855. Ohmic contacts 846 can be made of metalsilicide or other suitable material. Contacts 843, 844 and 845 extendfrom the ohmic contacts 846 to the respective metal segments 863, 864and 865, which are part of the first metal layer. Contacts 843, 844 and845 can be made of tungsten or other conductive material.

A local interconnection 849 connects conductive plug 809 to conductiveplug 843, thus connecting the photodetector 835 and the transistor 830.Any of the ohmic contacts 846 or the metal segments of the first metallayer 843, 844 or 845 can connect to other devices on a CMOS chip, suchas: resistors, capacitors, inductors, diodes or other transistors. Localinterconnection 852 connects the transistor's drain ohmic contact 846 toother devices on the integrated circuit 800.

Isolation dielectric 841 surrounds the sides of the body 840 of CMOStransistor 830. Inter-layer dielectric films 806 are deposited on top ofthe body 840 and gate 854. Gate spacer material 850 is a dielectric.

The body 840 of a CMOS transistor 830 can be fabricated at the same timeas the silicon core 805 of waveguide 804. The isolation dielectric 841of a CMOS transistor 830 can be fabricated at the same time as the sidecladding 841 made of dielectric on the side of the waveguide core 805.The insulator 803 of the SOI substrate 801 is used as the insulatorunderneath the CMOS transistor 830 and also as the bottom cladding ofthe photodetector 835. The inter-layer dielectric films 806 of the CMOStransistor 830 can be fabricated at the same time as the top claddingfilms 806 of the photodetector 835.

The conductive plugs 843, 844 and 845 extend from respective metalsegments 863, 864 and 865 to the drain, gate and source of the CMOStransistor. The conductive plugs 843, 844 and 845 are fabricated at thesame time as the conductive plugs 808 and 809 to the photodetector 835.The ohmic contacts 846 to the drain and source of the CMOS transistorcan be fabricated at the same time as the ohmic contacts 816 and 817 ofthe photodetector 835. The local interconnection 852 made between CMOStransistor 830 and other components on the chip can be fabricated at thesame time as the local interconnection 849 from the CMOS transistor 830to the photodetector 835.

The doping of the drain region 853 and the source region 855 of the CMOStransistor 830 can be performed at the same time as the doping of thedoped region 812 of the photodetector 835.

The polysilicon gate layer 854 of the CMOS transistor 830 can befabricated at the same time as the polysilicon mode converter 820 of thephotodetector 835.

FIG. 9 is a block diagram of an integrated, monolithic optoelectronicconverter, according to a preferred embodiment of the present invention.A monolithic CMOS integrated circuit 900 receives an optical signal 901from an optical transport such as a fiber optic cable 902. The fiberoptic cable 902 is connected to an optical coupler 903 on the chip 900.The optical coupler 903 sends the optical signal 904 to planar waveguide905. The optical coupler 903 can preferably connect to the chip 900 atthe surface of the chip, or alternately at the edge of the chip. Theoptical signal 904 propagates through waveguide 905 until the opticalsignal 906 reaches a waveguide photodetector 907 of the presentinvention. Waveguide photodetector 907 converts the optical signal 906to an electrical signal 908.

Electrical signal 908 is amplified by trans-impedance amplifier 909 andsent as amplified signal 910 to limiting amplifier 911. The output oflimiting amplifier 911 is digital signal 912, which is processed byclock recovery circuit 913 to recover a clock 914 encoded in the digitalsignal 912. The digital signal 912 is also processed by data recoverycircuit 915 to recover the digital data 916.

The encoding and recovery of clock and data signals into a digitalsignal can be accomplished by a variety of methods and techniques and iswell known to those skilled in the art, and need not be furtherdiscussed herein.

In alternate embodiments, the chip 900 could be smaller or larger interms of size and functionality. The chip 900 could send the electricalsignal 908 output, or the amplified signal 910 or the digital signal 912to another chip for further processing. The chip 900 could also containother digital circuitry, such as repeater, computational, digital signalprocessing or other systems, such as a digital to optical converter.

A particularly advantageous aspect of the present invention is that thefabrication of germanium on silicon waveguide photodetectors on SOI(silicon on insulator) substrates makes possible the integration of suchphotodetectors with CMOS circuits.

Another particularly advantageous aspect of the present invention isthat the germanium on silicon waveguide photodetector is built on thesame CMOS chip as the CMOS electronic circuits that process the outputof the photodetector, thus eliminating the need for hybrid circuits.Such integrated optoelectronic chips with small size photonic elementsof less than 1 μm in width, are much cheaper and faster than theequivalent hybrid chips.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade without departing from the spirit and scope of the invention.

1. A method for communicating signals to a semiconductor device usingoptical signals comprising: sending an optical signal to an input of agermanium on silicon waveguide photodetector located on a semiconductorchip, said germanium on silicon waveguide photodetector comprising: awaveguide comprising: a core comprised of a germanium on siliconheterojunction stack comprising: a silicon layer comprisingsubstantially silicon for conducting light, and a germanium layercomprising substantially germanium for conducting light; and a claddingcomprised of a plurality of dielectric materials; an optical input; afirst plurality of conductive contacts coupled to said germanium layer;and a second plurality of conductive contacts coupled to said siliconlayer; and, outputting an electrical signal through at least one of thesecond plurality of conductive contacts of the germanium on siliconwaveguide photodetector to an input of a semiconductor device located onthe semiconductor chip.
 2. A method for communicating signals to asemiconductor device using optical signals comprising: receiving anoptical signal at an input of a germanium on silicon waveguidephotodetector located on a semiconductor chip, said germanium on siliconwaveguide photodetector comprising: a waveguide comprising: a corecomprised of a germanium on silicon heterojunction stack comprising: asilicon layer comprising substantially silicon for conducting light, anda germanium layer comprising substantially germanium for conductinglight; and a cladding comprised of a plurality of dielectric materials;an optical input; a first plurality of conductive contacts coupled tosaid germanium layer; and a second plurality of conductive contactscoupled to said silicon layer; and, outputting an electrical signalthrough at least one of the second plurality of conductive contacts ofthe germanium on silicon waveguide photodetector to an input of asemiconductor device located on the semiconductor chip.
 3. A method forcommunicating signals to a semiconductor device using optical signalscomprising: sending an optical signal to an input of a germanium onsilicon waveguide photodetector located on a semiconductor chip, saidgermanium on silicon waveguide photodetector comprising: a waveguidecomprising: a core comprised of a germanium on silicon heterojunctionstack comprising: a silicon layer comprising substantially silicon forconducting light, and a germanium layer comprising substantiallygermanium for conducting light; and a cladding comprised of a pluralityof dielectric materials; an optical input; a first plurality ofconductive contacts coupled to said germanium layer; and a secondplurality of conductive contacts coupled to said silicon layer; and,outputting an electrical signal through at least one of the firstplurality of conductive contacts of the germanium on silicon waveguidephotodetector to an input of a semiconductor device located on thesemiconductor chip.
 4. A method for communicating signals to asemiconductor device using optical signals comprising: receiving anoptical signal at an input of a germanium on silicon waveguidephotodetector located on a semiconductor chip, said germanium on siliconwaveguide photodetector comprising: a waveguide comprising: a corecomprised of a germanium on silicon heterojunction stack comprising: asilicon layer comprising substantially silicon for conducting light, anda germanium layer comprising substantially germanium for conductinglight; and a cladding comprised of a plurality of dielectric materials;an optical input; a first plurality of conductive contacts coupled tosaid germanium layer; and a second plurality of conductive contactscoupled to said silicon layer; and, outputting an electrical signalthrough at least one of the first plurality of conductive contacts ofthe germanium on silicon waveguide photodetector to an input of asemiconductor device located on the semiconductor chip.
 5. A method forcommunicating signals to a semiconductor device using optical signalscomprising: sending an optical signal to an input of a germanium onsilicon waveguide photodetector located on a semiconductor chip, saidgermanium on silicon waveguide photodetector comprising: a waveguidecomprising: a core comprised of a germanium on silicon heterojunctionstack comprising: a silicon layer comprising substantially silicon forconducting light, and a germanium layer comprising substantiallygermanium for absorbing light; and a cladding comprised of a pluralityof dielectric materials; an optical input; a first plurality ofconductive contacts coupled to said germanium layer; and a secondplurality of conductive contacts coupled to said silicon layer; and,outputting an electrical signal through at least one of the secondplurality of conductive contacts of the germanium on silicon waveguidephotodetector to an input of a semiconductor device located on thesemiconductor chip.
 6. A method for communicating signals to asemiconductor device using optical signals comprising: receiving anoptical signal at an input of a germanium on silicon waveguidephotodetector located on a semiconductor chip, said germanium on siliconwaveguide photodetector comprising: a waveguide comprising: a corecomprised of a germanium on silicon heterojunction stack comprising: asilicon layer comprising substantially silicon for conducting light, anda germanium layer comprising substantially germanium for absorbinglight; and a cladding comprised of a plurality of dielectric materials;an optical input; a first plurality of conductive contacts coupled tosaid germanium layer; and a second plurality of conductive contactscoupled to said silicon layer; and, outputting an electrical signalthrough at least one of the second plurality of conductive contacts ofthe germanium on silicon waveguide photodetector to an input of asemiconductor device located on the semiconductor chip.